There is a constant demand for decreases in the size and increases in the operating speed of electronic equipment. In order to cope with this demand, it is necessary to decrease the size of electronic parts used in electronic equipment and to make the parts multi-functional. A representative example of electronic parts at which efforts are directed to produce decreases in size and multi-functionality are BGA (ball grid array) packages.
A BGA package typically includes a substrate having a semiconductor integrated circuit (IC) chip mounted on its top surface and an array of electrodes formed on its bottom surface. A rounded mass of solder, referred to as a solder bump, is attached to each of the electrodes. The BGA package can be connected to a printed circuit board, for example, by placing the BGA package atop a printed circuit board with each of the solder bumps of the package contacting a corresponding electrically conducting land of the printed circuit board, and then heating the BGA package and the printed circuit board so as to melt the solder bumps and solder them to the lands. Each of the solder bumps forms a minute soldered joint which mechanically and electrically connects the BGA package to the printed circuit board. The use of solder bumps is advantageous in that it enables a large number of uniform soldered joints to be simultaneously formed to all of the electrodes of a BGA package.
BGA packages can have a wide range of sizes and structures. When a BGA package has roughly the same planar dimensions as the integrated circuit chip mounted on its substrate, it is classified as a CSP (chip scale package). When a BGA package includes a plurality of IC chips, it is classified as a MCM (multi-chip module).
The solder bumps of a BGA package can be formed by a number of methods. One method employs solder balls. In this method, the electrodes of the substrate of the BGA, which may be in the form of a discrete substrate or a wafer to be cut into a number of substrates), are coated with a sticky flux, and previously formed solder balls are placed atop the coated electrodes and held thereon by the flux. The BGA substrate is then heated in a heating apparatus such as a reflow furnace to a temperature sufficient to melt the solder balls and form them into solder bumps on the electrodes.
Another method of forming solder bumps employs a solder paste, which comprises solder powder mixed with a pasty flux. In this method, solder bumps are generally formed on a wafer having lands in positions on which bumps are to be formed. A metal mask having holes of about the same size as the lands of a wafer is placed atop the wafer with the holes in alignment with the lands. Solder paste is then printed on the lands by forcing the solder paste through the holes in the mask using a squeegee. The wafer printed with the solder paste is then heated in a reflow furnace so as to melt the solder powder and form it into solder bumps on the lands of the wafer.
In the past, solder balls made of a Sn—Pb alloy were commonly used to form solder bumps for BGA packages. A Sn—Pb solder ball not only has excellent solderability with respect to the electrodes of a BGA substrate, but a Sn—Pb alloy, and particularly the eutectic composition, has a sufficiently low melting point that harmful thermal effects are not imparted to a BGA package or to a printed circuit board during soldering. At the same time, its melting point is sufficiently high that it will not melt at the temperatures produced inside electronic equipment by the heat generated by coils, power transistors, resistors, and other components during the operation of the electronic equipment.
When electronic equipment containing a BGA package formed using Sn—Pb solder balls becomes old and can no longer be used as desired or malfunctions, the equipment is usually not upgraded in performance or repaired but is almost always discarded. When such equipment is discarded, some portions of the equipment are capable of being reused or recycled. For example, plastics in cases, metals in frames, and precious metals in electronic parts are often recovered. However, a printed circuit board with components soldered to it typically cannot be reused. This is because the lands of a printed circuit board are metallically bonded to the solder, and it is difficult to completely separate the solder and the lands from each other. Therefore, discarded printed circuit boards are usually pulverized and then disposed of by burial in landfills.
If a printed circuit board which is disposed of by burial employs a lead-containing solder, such as a Sn—Pb solder, and if the printed circuit board is contacted by acid rain having a high pH, lead in the Sn—Pb solder can be dissolved out and mixed with rain water into underground water. If humans or livestock drink underground water containing lead over a long period, the lead can accumulate in the body and may cause lead poisoning. To avoid the environmental and health problems associated with the use of lead-containing solders, there is a movement in the electronics industry towards the use of so-called lead-free solders which do not contain lead.
Most lead-free solders are Sn alloys containing one or more of Ag, Cu, Sb, In, Bi, Zn, Ni, Cr, Co, Fe, P, Ge, and Ga. Among lead-free solders which are commonly used, those used for low to moderate temperatures include Sn—Bi based alloys, Sn—In based alloys, and Sn—Zn based alloys. However, these alloys have a number of problems for use as solders. For example, Sn—Bi based alloys easily undergo brittle fracture, Sn—In based alloys are expensive, and Sn—Zn based alloys easily undergo changes with time. Furthermore, when low temperature solders are employed in electronic equipment, they may melt when the temperature within a case for the equipment rises due to the generation of heat by heat-generating parts in the equipment. Even if the solders do not melt, their bonding strength may decrease enormously. Therefore, low temperature lead-free solders are limited to special applications.
Lead-free solders for medium temperature use (solders having a melting point somewhat higher than the Sn—Pb eutectic) include Sn—Ag based alloys, Sn—Cu based alloys, and Sn—Ag—Cu based alloys. Sn—Ag based alloys and Sn—Cu based alloys have problems with respect to wettability and resistance to heat cycles. Sn—Ag—Cu based alloys do not have the problems suffered by Sn—Ag based alloys and Sn—Cu based alloys and currently are the most widely used as lead-free solders.
When a Sn—Ag—Cu based alloy is used as lead-free solder to solder a component having a comparatively large bonding area, such as is the case with typical surface mounted parts or discrete parts, it is superior to conventional Sn—Pb solders even when it is subjected to impacts and heat cycles. However, as described below, a Sn—Ag—Cu based lead-free solder has problems when used to form solder bumps on minute electrodes, such as those of BGA packages.
So-called mobile electronic equipment such as mobile telephones, notebook computers, video cameras, and digital cameras often receives impacts from external forces. When such equipment contains a BGA package using a Sn—Ag—Cu based lead-free solder to connect the BGA package to a printed circuit board in the equipment, the soldered joints connecting the BGA package to the printed circuit board may sometimes be detached from the printed circuit board when subjected to an impact. When such detachment occurs, the electronic equipment can no longer function properly. Impacts sufficient to cause such detachment can easily occur during ordinary use of mobile electronic equipment. For example, a mobile telephone placed into a shirt pocket of a user may slip out and fall from the pocket when the user bends forward. Recent mobile telephones which have an e-mail function can easily be dropped when being operated by the user with one hand. When a notebook computer is carried in a briefcase, a significant impact can easily be applied to the computer if the owner accidentally drops the entire briefcase. In addition, it is not uncommon for a video camera or a digital camera to be dropped during use.
After solder bumps are formed on a BGA package, the package is subjected to a high temperature storage test. A high temperature storage test is a test which ascertains whether a BGA package undergoes deterioration in performance due to heat when electronic equipment containing the BGA package is left in a high temperature environment during use. The conditions of a high temperature storage test depend upon the manufacturer of electronic parts or the manufacturer of electronic equipment, but normally the test is carried out by leaving equipment for 12 to 24 hours in a high temperature atmosphere at 125-150° C. With a conventional Sn—Ag—Cu based lead-free solder, the solder bumps of a BGA package often undergo yellowing, i.e., the surfaces of the solder bumps become yellow, during a high temperature storage test. If the surfaces of solder bumps undergo yellowing in a high temperature storage test, when the solder bumps are subsequently inspected by image processing, accurate inspection cannot be performed. Thus, the yellowing may cause inspection errors.
Another problem of existing Sn—Ag—Cu based lead-free solders is that they do not have adequate resistance to heat cycles. When electronic equipment is operating, the electric current flowing through components of the equipment such as coils, power transistors, and resistors generates heat, and the interior of a case of the equipment rises in temperature. When the equipment is turned off and the current is stopped, heat is no longer generated by the components, and the interior of the case returns to room temperature. Each time electronic equipment is turned on and off in this manner, a heat cycle is repeated in which the temperature within the case rises and fall. Heat cycles also affect printed circuit boards and soldered joints in the equipment, causing thermal expansion and contraction of the printed circuit boards and the solder in the soldered joints connected to the printed circuit boards.
The coefficient of thermal expansion of the solder in soldered joints is significantly different from that of printed circuit boards, which are made of plastic. Therefore, when a rise in temperature takes place within electronic equipment, the amount of expansion of a soldered joint is constrained by the printed circuit solder to which it is connected, which has a lower coefficient of thermal expansion. On the other hand, when a soldered joint is subjected to a fall in temperature, its contraction is constrained by the printed circuit board. Therefore, as a result of electronic equipment being repeatedly turned on and off, soldered joints are exposed to heat cycles, and due to the stress in the soldered joints resulting from being constrained in elongation and contraction, metal fatigue takes place in the joints. Eventually cracking or fracture of the soldered joints can occur, and the soldered joints can then be detached from the printed circuit board. The same situation is found more or less between the soldered joints and the BGA package. For general use, a Sn—Ag—Cu based lead-free solder is greatly superior to a Sn—Pb solder with respect to resistance to heat cycles, but its resistance to heat cycles is still not sufficient for use when forming minute soldered joints for BGA packages.